1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method and a computer program.
2. Description of the Related Art
For realizing high speed wireless LAN communication, IEEE 802.11n standard that utilizes MIMO technology has been proposed. In IEEE 802.11n standard, wireless communication apparatus can select 20 MHz or 40 MHz as the communication bandwidth. In IEEE 802.11a, which is in the same 5 GHz band as the IEEE 802.11n, a single communication channel is stipulated to 20 MHz bandwidth. However, in practice, a wireless communication apparatus conformed to IEEE 802.11a standard conduct communication using a 26 MHz bandwidth. Therefore, interference is occurred not only with the same communication channel, but also among the adjacent communication channels. This is explained using the FIG. 8.
FIG. 8 is a figure showing the channel interval, as well as a figure for explaining the image of used frequency spectrum. Frequency spectrum 801 shows the frequency spectrum used for conducting communications using the channel N. Since the actual bandwidth of the frequency spectrum 801 is 26 MHz, there are frequency spectrum parts that overlap with the N+1 and N−1 channel frequency spectrum. Therefore, when there are two wireless communication apparatuses located within proximity, not only when the communication channels are equal, but also when communication channels are adjacent to each other, the interference is occurred. As a result, in carrier-sense before the data transmission, this may be seen as existence of a carrier and therefore the packet communication from one of the wireless communication apparatuses may be restrained.
Even when the packet transmission is restrained as mentioned above due to the interference, there may be cases that it is not a problem if the communication peer receives all the packets at the end. However, when data transmission is conducted within limited timeframe such as in video streaming, throughput is decrease due to the transmission restrain occurred by the adjacent channel communication; as a result, image quality decreasing effects such as image disturbance and delay is occurred.
As examples, the examples given in FIG. 9 and FIG. 10 are considered. FIG. 9 shows a figure for explaining the case of two wireless communication apparatuses conducting communication using two adjacent channels. FIG. 10 shows a figure for explaining the case of two wireless communication apparatuses conducting communication using the same communication channel. In FIG. 9, a Web camera 901 and a PC 902 are considered to be conducting video stream communication. The Web camera uses the channel N and the PC 902 uses the channel N+1. The Web camera 901 and the PC 902 is in adjacent, the transmission restrain cannot be ignored, which could lead to a significant through-put decrease. Moreover, as shown in the example of FIG. 10, even in case the Web camera 1001 and the PC 1002 is conducting communication within the same network, due to interference, decrease in the quality of the video stream is occurred.
To prevent such a throughput decrease, in Japanese Patent Laid-Open No. 2004-260528, when the interfering wave of the adjacent channel is detected, a technology for narrowing the bandwidth of the band pass filter which is used to pass the signal of the currently used channel, has been proposed. In Japanese Patent Laid-Open No. 2005-333510, when interference due to the adjacent station is detected, changing the channel to a channel that does not affected by the adjacent station, has been proposed. In Japanese Patent Laid-Open No. 2006-217491, when the carrier of the adjacent station is detected, a technology for changing the antenna directivity so that the antenna does not receive carrier of the adjacent station, has been proposed. In particular, there are also cases such as changing the communication channel in order to avoid interference as in Japanese Patent Laid-Open No. 2005-333510. However, in case of Wireless LAN, it is necessary to change the channel at the access point side; therefore, changing from the terminal side is not possible. Further, in case channel is changed while communicating, the communication will be cutoff. Further, when the antenna directivity has been changed, as in Japanese Patent Laid-Open No. 2006-217491, interferences due to other stations can be avoided, however avoiding interferences due to station itself cannot be conducted. Furthermore, in apparent when the signals of the other stations are not visible, the station itself will conduct communication regardless whether the other stations are communicating or not; therefore, collation occurs in the wireless domain, resulting increase in the communication error frequency.
The present invention proposes a technology that reduces the through-put decrease, even in case there are interferences with the other wireless communication apparatuses.